Speaker Fuse

Bill Plemmons · Post by **Bill Plemmons** » 31 Mar 2004 7:35 am

I recently saw an early 70's Evans amp that had been modified with a 5 amp speaker fuse. Bose recommended inline fuses for my 901 speakers. This looks like a good idea and I'm considering installing inline speaker fuses in all my amps. Anyone done this? How do I calculate the fuse rating?

Bill

Ray Minich · Post by **Ray Minich** » 31 Mar 2004 7:44 am

Speaker fusing is not the cure-all that one would expect. Usually the fault current that it takes to blow the fuse also hurts the speaker, and/or the output transistors. My books all give it a 2 out of 10 on the protection scale. It can be done though, I'll look in my archives and see what the "fuse coordination" recommendations are for this service.

BTW, years back on a bi-amped PA system we used a 1/2 ohm 500 watt resistor in series with the speaker to protect the Crown amp's output section in case the speaker blew "shorted".

John Floyd · Post by **John Floyd** » 31 Mar 2004 11:13 am

I've seen single element 12 Volt Auto lamps or bulbs in series with a horn to protect it. They seem to work, becuase I bought a pair of speakers once and the bulbs were blown and the horns were ok.

Keith Hilton · Post by **Keith Hilton** » 31 Mar 2004 7:13 pm

I agree with Ray. DC current is more dangerous than AC and it is possible for speakers to get DC surges. I hope this isn't my old amp. I blew 3 speakers with a volume less than 2 on the E9th neck. Finally installed a resistor and reconed it as a bass speaker and sold the amp. Looking back, I suspect a DC surge the most.

ajm · Post by **ajm** » 1 Apr 2004 7:32 pm

I have a Carvin XV112 tube amp with a fuse in series with the speaker. I always thought that if you ever blew the fuse and opened up the line that you'd probably blow the output transformer as well, since you'd be operating into an open load.

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Artie McEwan

C Dixon · Post by **C Dixon** » 1 Apr 2004 8:00 pm

AJM,

Theoretically that is true. But consider the following:

That is ONLY true IF a player continued to play through the amp at full power. What player would do that if he got no sound? So the engineers took the lessor of two evils.

Keep in mind, that unless there is full signal present at the output stages there is little to no current flowing through the output transformer, so nothing would blow if there was no load, IF there was no continuous full power signal. (note: assumes Class AB)

Most transformers can easily take a few moments of full power with no load before the windings start to overheat.

Now on a transistor amp it is just the opposite. No load hurts nothing. The real problem here is a load that approaches a "short" such as going lower than say 1 or 2 ohms. But even here, UNLESS there is signal present, no problem.

carl